FIG. 1 shows a conventional signal transmitter having two types of data paths of an in-phase signal I and a quadrature-phase signal Q. Signals of the paths I and Q of the signal transmitter are first respectively processed by a mixer 12 and a mixer 14, and then are modulated by a carrier signal generated by a local oscillator 16. An adder 18 accumulates the two modulated signals. Since un-ideal factors exist in circuits, even when no baseband signals are provided by subsequent circuits to the paths I and Q, noise (so-called carrier leakage) is created at an output end of the adder 18.
In order to avoid negative influences created by the carrier leakage, a power upper limit (e.g., −30 dBm) of a carrier leakage signal is clearly defined in most communication specifications. Therefore, a calibrating circuit for detecting and reducing the carrier leakage is needed at the output end of the signal transmitter. The calibrating circuit commonly comprises a power detector or a mixer-type detector, an analog-to-digital converter (ADC), and a determining circuit.
Taking calibrating the path I as an example, a switch 11 for connecting the path I to the adder 18 is set closed, and a switch 13 for connecting the path Q to the adder 18 is set open. When the carrier leakage power created by the path I at the output end of the signal transmitter is converted to a voltage value by the power detector or the mixer-type detector, the voltage value is converted to a digital value by the ADC. The determining circuit of the calibrating circuit generates an analog calibration signal fed to the path I according to the digital value, and adds direct current (DC) voltages to the calibration signal to reduce signal power of the carrier leakage signal. The determining circuit continuously calibrates the foregoing calibration signal until the signal power of the carrier leakage signal created by the path I is lower than the threshold defined in the specifications. After the calibration of the path I completes, the calibration circuit iterates the foregoing steps to calibrate the path Q.
In addition to the signal transmitter in FIG. 1, most conventional circuits adopt a power detector or a mixer-type detector to measure the signal power. The power detector and the mixer-type detector convert the signal power of a to-be-detected signal to a voltage value, and an input signal of the power detector or the mixer-type detector is directly proportional to an output signal. Therefore, influences on accuracy of a detection result created by un-ideal factors of circuits of the power detector and the mixer-type detector become larger as the input signal power gets smaller. Unless an expensive, accurate power detector or mixer-type detector is utilized, a signal having low power, like the carrier leakage, is rather hard to be accurately detected.
In addition, there are DC errors occurring in the ADC for further converting an output voltage of the power detector or the mixer-type detector to the digital value. Therefore, the conventional power detector regarding the voltage value as a comparing reference value cannot accurately determine the signal power, and may even create calibration errors in subsequent circuits.